The problem that this research addresses is the greater likelihood of mental illness suffered by females beginning in adolescence. The subjects are early adolescents, since there is accumulating evidence that this is the most critical phase of adolsecence, particularly for female development. This study systematically investigates biopsychosocial development in early adolscent males and females. The parents of these young people are also studied. The deisgn includes two cohorts of early adolescents, each followed for three years beginning at sixth grade. In each cohort, 100 males and 100 females have been selected. Two control groups, a retest control group and a dropout control group will also be utilized. Information on the family, school issues, peers, and personal interests and issues including pubertal development is obtained in a semistructured interview with each child twice annually. Questionnaires assessing psychological and cognitive development are also administered in small groups twice annually. Parents are interviewed when their children are in sixth and eighth grades. Annual achievement data are obtained from the schools. This study will provide basic developmental data currently lacking on early adolescents. Specific questions about the development of sex-related differences will be addressed. The results will be significant for our understanding of development and how we might most effectively intervene with youth this age.